There are a wide variety of sensor technologies available. While many of these technologies may be used to sense one or more gasses, each comes with one or more drawbacks or tradeoffs as compared with the present invention, most are not suitable for wide-spread portable applications for a variety of reasons. Many sensors are physically too large for portability. Others consume too much power to be reasonably operated by a battery. Others operate at very high temperatures making them difficult to use in portable applications. Still others are too expensive for wide-spread use, and many require training to use properly. A survey of existing mainstream sensor technology is outlined below.
Chromatography
A chromatograph is an apparatus that separates a complex mixture into individual components. A mixture of gas components is injected into a chromatograph column, where the components travel down the column at different rates, reaching the end of the column at different times. A detector is positioned at the end of the column to quantify the concentrations of individual components of the mixture as they reach the end of the column.
The apparatus used in gas chromatography consists of four basic components: (1) a carrier-gas supply and flow controller, (2) a sample inlet system providing a means for introduction of the sample, (3) the chromatographic column and associated column oven, and (4) the detector system. The column is the heart of the gas chromatograph. Typically the column is a glass or metal tube of that is 6 to 13 mm in diameter and 1 to 2 meters in length. The chromatic column is filled with a carrier gas, most commonly nitrogen or helium, though other gases such as carbon dioxide, argon, xenon, and hydrogen are occasionally used.
The detector produces a response that is proportional to component that is separated by column and is located at the end of the column. Different detectors may be utilized dependent upon the analyte of interest. Any one of many different types of detectors could be used, such as an ion mobility spectrometer (IMS), mass spectrometer, photo ionization detector (PID), flame ionization detector (FID), thermal conductivity detector (TCD), electron capture detector (ECD), flame photometric detector (FPD) or far UV absorbance detector (FUV).
Chromatography is a well known and accurate technique for identifying constituents in a gas mixture. However, these units tend to be large and expensive, and specialized training is required to use them effectively.
Ion Mobility Spectrometry (IMS)
The gaseous sample to be analyzed enters a spectrometer, where it is ionized by a radioactive source. The resulting positive and negative charged species are then accelerated over a short distance and the time-of-flight is determined. The IMS differs from mass spectrometer in that it operates under atmospheric conditions and does not need large and expensive vacuum pumps. Because of this, IMSs can be miniaturized.
Like Chromatography, IMS is well know and accurate. However, IMS units are large and expensive, and require trained personnel to make a measurement.
Mass Spectrometry
The principle of the mass spectrometer is similar to the ion mobility spectrometer, except a vacuum is required. The sampled gas mixtures are ionized, and charged molecular fragments are produced. These fragments are sorted in a mass filter according to their mass to charge ratio. The ions are detected as electrical signals with an electron multiplier or a Faraday plate. Low mass ions are displayed as a vertical line at the left end of a scale while heavy ions are displayed towards the right. The length of a line represents the quantity of that ion in the gas mixture.
Mass Spectrometry is well know and accurate. However, IMS units are large and expensive, and require trained personnel to make a measurement.
Electrochemical Sensors
There are a wide variety of electrochemical sensors, which can be classified in two basic groups called potentiometric, in which a voltage measured and amperometric, in which a current is measured. These sensors consist of a casing that contains a collection of chemical reactants (electrolytes or gels) in contact with the surroundings through two terminals, an anode and a cathode. For gas sensors, the top of the casing has a membrane which can be permeated by the gas sample.
The presence of the gas to be detected causes oxidization to take place at the anode and reduction to take place at the cathode. A current is created as the positive ions flow to the cathode and the negative ions flow to the anode. Gases such as oxygen, nitrogen oxides, and chlorine, which are electrochemically reducible, are sensed at the cathode while electrochemically oxidizable gases such as carbon monoxide, nitrogen dioxide, and hydrogen sulfide are sensed at the anode. Potentiometric measurements are performed under conditions of near-zero current. Amperometric sensors are usually operated by imposing an external cell voltage sufficiently high to maintain a zero oxygen concentration at the cathodic surface. Sensitivity of amperometric sensors is generally better than potentiometric sensors.
Electrochemical sensors are wide-spread, but are typically too large and require too much power for portable applications. In addition, these sensors typically utilize toxic materials that are not suitable for portable applications.
Fiber Optic Sensors
Fiber optic sensors are a class of sensors that use optical fibers to detect chemical contaminants. Light is generated by a light source and is sent through an optical fiber. The light then returns through the optical fiber and is captured by a photo detector. Some optical fiber sensors use a single optical fiber while others use separate optical fibers for the light source and for the detector.
There are three general classes of fiber optic sensors The first type is completely passive. A spectroscopic method can be used to detect individual types of contaminants. This method involves sending a light source directly through the optical fiber and analyzing the light that is reflected or emitted by the contaminant. A second class of fiber optic sensors consist of a fiber optic sensor with a chemically interacting thin film attached to the tip. This film is formulated to bind with certain types of chemicals. Contaminant concentration can be found by measuring the color of the thin film, the change in refractive index, or by measuring the fluorescing of the film. The third type of fiber optic sensors involves injecting a reagent near the sensor. This reagent reacts either chemically or biologically with the contaminant. The reaction products are detected to give an estimate of the contaminant concentration.
These types of sensors have the advantage of power. Several types have no moving parts. They are also capable of detecting various chemicals at very low concentrations. However, they are limited by the ability to transmit light through the optical fiber over long distances. Some organic pollutants are not easily differentiated using UV-visible spectroscopy.
Colorimetry
Colorimetric devices work by analyzing the color of contaminated water that has been mixed with a particular chemical reagent. Pre-measured, unit-dose reagent that react with water samples are available for sale. To test water samples, the pocket colorimeter compares a reacted sample with a sample blank and yields results in concentration units.
Pocket colorimeter test kits are portable and are simple to use. They provide visual evidence of a gas detection event, and they are not prone to interferences. However, these devices have limited chemical sensitivity to individual VOCs. They also need actual water samples for testing, and cannot therefore be used in situ. Most kits do not meet U.S. EPA method requirements and may not be used for compliance monitoring.
Infrared Sensors
Infrared sensors can be used to detect gases, which, in general, have unique infrared absorption signatures in the 2-14 um range. The uniqueness of the gas absorption spectra enables identification and quantification of chemicals in liquid and gas mixtures with little interference from other gases. These devices are typically comprised of a source of infrared radiation, a detector capable of seeing the infrared radiation, and a path between the detector and source that is exposed to the gas being detected. When gas in the path absorbs energy from the source, the detector receives less radiation than without the gas present, and the detector can quantify the difference.
Mass Sensor—Surface Acoustic Wave Sensors/Portable Acoustic Wave Sensors
Surface Acoustic Wave Sensors (SAWS) are small miniature sensors used to detect VOCs. A SAW device consists of an input transducer, a chemical absorbent film, and an output transducer on a piezoelectric substrate. The piezoelectric substrate is typically quartz. The input transducer launches an acoustic wave which travels through the chemical film and is detected by the output transducer. The device runs at a very high frequency, generally about 100 MHz. The velocity and attenuation of the signal are sensitive to the viscoelasticity as well as the mass of the thin film which can allow for the identification of the contaminant. Heating elements under the chemical film can also be used to desorb chemicals from the device. A signal pattern recognition system that uses a clustering technique is needed to identify various chemicals.
SAWS have been able to distinguish organophosphates, chlorinated hydrocarbons, ketones, alcohols, aromatic hydrocarbons, saturated hydrocarbons, and water. They are small, low power, have no moving parts other than the high-frequency excitation, and are known to have good sensitivity to various chemicals, able to detect chemicals in very low concentrations. However, they are generally not able to discriminate among unknown mixtures of chemicals.
Metal-Oxide Semiconductor Sensors
Metal oxide sensors are the among the earliest and most popular sensor element in sensor arrays. For example, oxygen sensors used in every automobile are metal oxide sensors. A metal oxide sensor is an n-type inorganic semiconductor, such as tin oxide, doped tin oxide derivatives, zinc oxide, or iron oxide. These materials are heated to temperatures of 300 C to 550 C and used as a two-terminal resistive device. The response arises from the reduction of the gas species at the surface of the semiconductor which increases the electron carrier concentration, resulting in higher conductivity. The gas species is consumed as the sensor operates.
The sensor is traditionally constructed using a ceramic support tube containing a platinum heater coil and the metal oxide is coated onto the outside of the ceramic tube with the appropriate electrical leads connected to the film. In recent decades, newer implementations of the metal oxide sensor have been constructed with planar configurations using conventional micro-fabrication techniques. While using micro-fabrication methods is advantageous, there is a significant challenge with integration of multiple sensor elements into a single array. The procedure can be difficult and expensive due to extensive subtractive processes arising from materials compatibility across the different elements. In addition, there are complicating issues related to its high operating temperature.
Another disadvantage of the metal oxide is power consumption. Because the sensing response depends directly on the reduction reaction, which is very sensitive to the temperature, the sensor must be heated to at least 300 C. Generating such high temperatures is consumption, typically around 800 mW. Yet another disadvantage is a general lack of discrimination since the combustion mechanism limits the sensitivity of the device and is not reliant on the chemistry of the gas species itself.
Despite the these disadvantages of power consumption and integration, the metal oxide is by far the most commonly used gas sensor in commercially available electronic noses. The sensor is rugged and rather versatile and is already used in many other industrial settings. Of those that are often used for industrial applications, the tin oxide sensor doped with palladium or platinum is the most popular. However, the use of metal oxides continues to pose a fundamental problem of difficult integration.
Polymer-Absorption Chemiresistors
The concept of using polymeric absorption to detect the presence of chemicals in the vapor phase has existed for several decades. These polymer-absorption sensors, also called chemiresistors, consist of a chemically sensitive absorbent that is deposited onto a solid phase that acts as an electrode. When chemical vapors come into contact with the absorbent, the chemicals absorb into the polymers, causing them to swell. The swelling changes the resistance of the electrode, which can be measured and recorded. The amount of swelling corresponds to the concentration of the chemical vapor in contact with the absorbent. The process is reversible, but some hysteresis can occur when exposed to high concentrations. Several companies and organizations have developed chemiresistors, but the specific attributes and types of absorbents, which are generally proprietary, vary among the different applications.
Chemiresistors are attractive because they are small, low power devices that have no moving parts and have good sensitivity to various chemicals. However, these types of sensor generally react to a broad array of analytes, making it difficult to identify a specific gas. Some polymers react strongly to water vapor and moisture. Although the reaction with the analyte is designed to be reversible, the signal may experience hysteresis and a shift in the baseline over time when exposed to chemicals.
Conducting Polymer Sensors
A conducting polymer sensor is based on a polymer material possessing electrical properties that can selectively absorb specific odorants. Exposure to a gas analyte induces changes in the electrical behavior of the sensor. The sensor response arises from intermolecular interactions between the sensor and the analyte. These interactions are the result of hydrogen bonding, dipole-dipole or dipole-induced dipole dispersions, and hydrophobic forces. For this reason, conducting polymer sensor array are considered the most similar to the olfactory sensor of a biological nose.
The most popular materials is polypyrrole, a key feature of the polymer of which is the repeating and alternating arrangement of double bonds throughout the molecule. This feature, known as a conjugated pi electron system, gives rise to the electrical behavior. Reduction and oxidation of the polymer can subsequently modify its charge conducting behavior and it is surmised that the sensor response may result from reduction and oxidation processes or similar interactions involving partial charge transfer. Polypyrrole is most often deposited using electrochemical polymerization in order to avoid solvent compatibility issues. The properties of the film are strongly affected by the growth conditions but with careful control, purified and reproducible films can be achieved.
Conducting polymer sensors have several advantages. In general, they respond to a broad range of organic vapors and there is a large spectrum of materials that can be synthesized allowing for a wide range of selectivity. There are synthesized using relatively low cost materials and the sensor element can be fabricated with an attractive form factor. Unlike metal oxide sensor, conducting polymer sensors operate at room temperature, reducing power consumption and providing for longer lifetime.
A major disadvantage of a conducting polymer sensor is they tend to be sensitive to humidity and show a long-term drift in their performance. Further, they do not generally display a high specificity to individual gases. However, these polymers can be chemically tailored to enhance differences to response to classes of molecules.
ChemFET Sensors
A Chemically sensitive FET, also known as a ChemFET, is the structural analog of a MOSFET, with the polysilicon gate replaced with a chemically sensitive layer. The gate is chosen such it reacts with the gaseous species to cause a work function shift that can be detected through FET operation. Though ChemFETs are amenable to CMOS integration, in practice fabrication is greatly complicated since most polymers are electrochemically deposited.
Therefore, it would be desirable to provide a sensor that overcomes the drawbacks of the prior art to make it suitable for low-cost, low-power, wide-spread, and portable applications.